1. Field of the Invention
The present invention relates to a detection method for detecting a position of an image formed by a tandem color recording device having an electrophotographic system.
2. Related Art
A recording device having an electrophotographic system performs charging, exposing, developing, and transferring steps to form a color image on the surface of a recording sheet by using color particles and a fixing step to fix the color image on the recording sheet. Toner that is powder for electrophotograph is used as the color particles.
In the charging step, the entire surface of a photosensitive member is charged. In the subsequent exposing step, areas on the photosensitive member are exposed to light to remove the charge therefrom. These steps generate a contrast in potential between the charged areas and discharged areas on the surface of the photosensitive member, thereby forming an electrostatic latent image.
Next in the developing step, toner particles are charged, and the electrostatic latent image is developed using the charged toner particles. Methods for charging toner include dual-component development in which carrier beads are used and single-component development in which the toner particles are tribocharged by friction generated between the toner particles and components of the recording device. A method called bias development is widely used for developing electrostatic latent images.
In the bias development, a bias voltage is applied to a developing roller. Through the effect of an electric field generated between the developing roller and the electrostatic potential developed on the surface of the photosensitive member, the charged toner particles are separated from developer (a mixture of toner particles and carrier beads) on the surface of the developing roller and are transferred onto the electrostatic latent image formed is on the surface of the photosensitive member, thereby forming a visible image.
A latent image potential, that is, the potential of the electrostatic latent image, may be a charge potential or the discharge potential described above. Generally, the method using the charge potential as the latent image potential is called normal development, while the method employing the discharge potential is called reverse development. The charge potential or discharge potential not being used as the latent image potential is called background potential. The bias potential of the developing roller is set between the charge potential and the discharge potential, and the difference between the bias potential and the latent image potential is called developing potential differential. Similarly, the difference between the bias potential and the background potential is called background potential differential.
A background potential differential that is too large tends to generate thin spots and defects on the trailing edge of the image in relation to the rotational direction of the developing roller. In addition to the background potential differential, deterioration of the developer and irregularities in other developing conditions may also lead to such thin spots and defects in the trailing edge of the image.
An electrophotographic device capable of recording multicolor images, such as a tandem color electrophotographic device, uses a plurality of image-forming units to form an image for each color (separated color). Multicolor images are formed by superimposing the plurality of images in each color, transferred onto a recording medium, and fixed on the recording medium.
However, misalignment in the transferred images may be caused by irregularities in the various mechanical systems of the tandem color recording device, such as eccentricity of the photosensitive member, positional or pitch deviation in the mounting positions of the exposure devices, speed variations between the plurality of photosensitive members, and skew or speed fluctuations in the transfer belt. Such a misalignment in the transferred images causes image misalignment. Alignment errors in the electrostatic latent images may result from irregularities in the surface of the polygon mirror provided in the exposure device and the like, which in turn may also cause image misalignment.
U.S. Pat. No. 5,287,162 proposes a technology for preventing this type of image misalignment (color registration errors). According to this technology, each image-forming unit is used to form a color registration detection pattern (patch in chevron shape) in each separated color on the surface of an intermediate transfer member. Photoreceptors detect the position of the detection patterns. Then, image misalignment is corrected based on detection signal from the photoreceptors.